<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Document</title>
    <style>
        body {position:relative;}
        .indicator {position:absolute; top:0; bottom:0; left:0; right:0; background:linear-gradient(to right top,teal 50%,transparent 50%) no-repeat -10px 0; background-size: calc(100% + 20px)  calc(100% - 100vh + 5px); z-index:1; pointer-events:none; mix-blend-mode:darken;}
        .indicator::after {content:""; position:fixed; top:5px; bottom:0; left:0; right:0; background:#fff; z-index:1;}
        .main {height:3000px;}
    </style>   
</head>
<body>
    <div class="indicator"></div>
    <div class="main">
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    </div>
</body>
</html>